Methodology

Every calculator on this site follows the same shape:

1. Rules become inputs. Whatever the law or regulation actually keys off — income, household size, distance flown, years of age — becomes a field in the calculator. We don’t ask for anything the rule doesn’t need.
2. A rules engine, not a guess. The thresholds, caps and rates come from the primary source listed on that tool’s page, encoded as data rather than hard-coded into the display logic — the same numbers a human could look up and check by hand.
3. Every non-trivial calculation is tested against a worked example from the official source (an IRS example, a CMS example, a regulator’s own case) before the tool is published, not just against numbers we made up ourselves.
4. The result shows its work. Where it’s practical, a calculator shows the intermediate steps — not just a final number — so you can spot where a figure comes from.

What every result assumes, unless the tool says otherwise

• Figures are illustrative, not guaranteed. Anything that depends on an assumption you don’t control — an investment return, an inflation adjustment not yet published — is labeled as an assumption, and you can usually change it.
• Amounts are approximate. Calculators round to a sensible precision; they are not a substitute for a line-by-line filing or an official determination letter.
• One rule at a time. A calculator models the rule it’s named after, not your entire tax or benefits situation. Other rules can interact with it in ways a single calculator won’t capture.
• Jurisdiction and tax year matter. Every tool states which year’s rules and which country’s law it uses. Using it for a different year or place will give you the wrong answer.

None of this is personalized advice — see the disclaimer. If a calculator’s assumptions matter to a decision you’re making, check them against the sources listed on that tool’s page, or with a licensed professional.